The present invention is directed to paper products exhibiting markedly improved dry strength properties. These products are prepared by incorporating into the paper product a chitin-based compound comprising a chitosan or a novel graft copolymer of certain acrylic monomers or diallylic monomers onto a chitosan substrate. These chitin compounds are incorporated into the paper product during the papermaking process, preferably by adding them to the aqueous cellulosic pulp dispersion.
It is a well-accepted fact that it is desirable in many applications to have paper products with good dry strength. In addition, it is well known that the paper industry has a strong movement underway to reduce the basis weight of paper, especially that of publication-grade paper. Reduced basis weight in paper would correspondingly reduce mailing cost. Dry strength aids are needed for lighter weight paper because as the basis weight is lowered, the dry strength of the paper also decreases. By using dry strength additives to maintain the strength of the lower basis weight paper, the production costs are reduced since less pulp and power are needed to make an equivalent sheet.
In the past, natural polymers such as guar and locust bean gums and the native and modified starches have been the most commonly used dry strength additives. The performance of these natural polymers is difficult to control and hence somewhat inconsistent. In addition, the use of starches involves lengthly preparation procedures and they are not well retained by the fibers without the use of additional costly additives. However, because of their low cost and availability, these compounds have heretofore been used despite their disadvantages.
More recently, several synthetic dry strength resins have appeared on the market. These compounds are basically modified polyacrylamides or modified cationic starch derivatives. These compounds, while somewhat effective under normal conditions, do not maintain paper strength at lower basis weight and they do not function well in alkaline media. The requirement of functioning well in an alkaline system is important since there is a desire in the paper industry to change from the present acid (pH 4 to 5.5) system of papermaking to neutral or alkaline (pH 7 to 8.5) system. The acid system is detrimental to machine parts and results in a paper sheet that becomes brittle and yellow with age. Another advantage of an alkaline system is that an inexpensive pigment, such as calcium carbonate, can be used instead of the more expensive titanium dioxides and aluminum oxides.
Therefore, it is an object of this invention to produce a dry strength additive which works well in low basis weight paper, gives consistent performance and performs well in both alkaline and acidic systems.